The present invention is concerned with optically active, ferroelectric, smectic photo-cross linkable liquid crystals, liquid crystalline mixtures which contain such compounds as well as their use in the cross-linked state in optical components.
Photo-cross linkable liquid crystals provided with an appropriate amount of a photoinitiator can be orientated on a substrate or in a cell by suitable orientating layers or in a field and then in this state can be cross-linked by irradiation with light of a suitable wavelength. The structure thereby produced is preserved even at high temperatures. Such layers can be, for example, parts of hybrid layers as are described in Swiss Patent Applications CH 2016/94 and CH 2017/94. Thus, optical components, such as, for example, wave guides, optical screens and filters, components having piezoelectric properties and those having non-linear optical (NLO) properties, etc., can be produced. Such optical components are used, for example, in frequency doubling (SHG).
Further properties such as, for example, the birefringence, the refractive index, transparency, etc. must fulfil different requirements depending on the field of use. For example, materials for frequency doubling (SHG) should have a strong coupling between the direction of maximum hyperpolarizability and the polar axis in order that the degree of efficiency of the signal doubling is high.
Moreover, the photo-cross linkable liquid crystals must have a good chemical and thermal stability, good solubility in usual solvents and a good stability towards electric fields and electromagnetic radiation. Furthermore, they should have a suitable mesophase from about 25.degree. C. to about 100.degree. C., especially from about 25.degree. C. to about 80.degree. C., for example they should have a broad chiral smectic mesophase for the applications referred to above.
Since liquid crystals are usually used as mixtures of several components, it is important that the components have a good miscibility with one another. Components which in mixtures bring about a frequency doubling or lead to optical activity are usually not liquid crystalline themselves. They therefore usually have the disadvantage that they lead to a large decrease of the phase transition temperatures and accordingly can be used only in low concentrations. This leads to a small SHG degree of efficiency. Mixtures consisting of photo-cross linkable liquid crystals and non-photo-cross linkable liquid crystals and/or optically active (chiral) additives permit a relaxation of the non-cross lin.kable components and thereby lead to a diminishing of the SHG signal. On the other hand, networks in which the active components are incorporated permit high concentrations of these active additives. At the same time, the orientating relaxation is prevented by the fixation in the network. Blue or green light can be produced by using in such networks colouring substances which absorb at the wavelength of the SHG light generated by a laser diode. Such networks have the disadvantage that usually they do not have sufficient long-term stability.
There is accordingly the need to produce photo-cross linkable components which satisfy the aforementioned requirements, can be structured, have an excellent thermal stability and long-term stability and a high SHG degree of efficiency.